This study is designed to determine the profile of physiologic, psychomotor, and self-reported effects of infusion rate of intravenously administered morphine, a drug with known abuse liability, in human participants. Three doses of morphine (0, 5, and 10 mg/70 kg, IV) will be administered under three infusion rates (2 min. bolus, 15 min. and 60 min.) Outcome measures will include a variety of self-report questionnaires, independent observations of opioid agonist and withdrawal effects, physiologic measures, as well as psychomotor performance. This study will provide an empirical investigation of whether the rate of onset of morphine's effects contributes to morphine's reinforcement value.